Chapter 17 Additional Aspects of Aqueous Equilibria Subhash Goel South GA State College Douglas, GA...

Chapter 17

Additional Aspects of Aqueous Equilibria

Subhash GoelSouth GA State College

Douglas, GA© 2012 Pearson Education, Inc.

AqueousEquilibria

The Common-Ion Effect

• Consider a solution of acetic acid:

• If acetate ion is added to the solution, Le Châtelier says the equilibrium will shift to the left.

CH3COOH(aq) + H2O(l) H3O+(aq) + CH3COO(aq)

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The Common-Ion Effect

“The extent of ionization of a weak electrolyte is decreased by adding to the solution a strong electrolyte that has an ion in common with the weak electrolyte.”

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Exercise 1 Calculating the pH When a Common Ion is Involved

What is the pH of a solution made by adding 0.30 mol of acetic acid and 0.30 mol of sodium acetate to enough water to make 1.0 L of solution?

AqueousEquilibria

The Common-Ion Effect

Exercise 2 : Calculate the fluoride ion concentration and

pH of a solution that is 0.20 M in HF and 0.10 M in HCl. Ka for HF is 6.8 104.

[H3O+] [F][HF]

Ka = = 6.8 104

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Buffers• A buffer resists changes in pH because it

contains both acid to neutralize added OH- ions and a base to neutralize added H+ ions.

• Solutions of a weak conjugate acid–base pair fulfill this requirement and acts as buffers.

• They are particularly resistant to pH changes, even when strong acid or base is added.

• Examples: CH3COOH and CH3COO- ion solution or NH4

+ ion and NH3 solution or HF and F- ions solution.

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Buffers

If a small amount of hydroxide is added to an equimolar solution of HF in NaF, for example, the HF reacts with the OH to make F and water.

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Buffers

Similarly, if acid is added, the F reacts with it to form HF and water.

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Buffer Calculations

Consider the equilibrium constant expression for the dissociation of a generic acid, HA:

[H3O+] [A][HA]

Ka =

HA + H2O H3O+ + A

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Buffer Calculations

Rearranging slightly, this becomes

[A][HA]

Ka = [H3O+]

Taking the negative log of both side, we get

[A][HA]

log Ka = log [H3O+] + log

pKa

pHacid

base

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Buffer Calculations

• SopKa = pH log

[base][acid]

• Rearranging, this becomes

pH = pKa + log[base][acid]

• This is the Henderson–Hasselbalch equation.

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Henderson–Hasselbalch Equation

Example 3

What is the pH of a buffer that is 0.12 M in lactic acid, CH3CH(OH)COOH, and 0.10 M in sodium lactate? Ka for lactic acid is 1.4 104.

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pH Range

• The pH range is the range of pH values over which a buffer system works effectively.

• It is best to choose an acid with a pKa close to the desired pH.

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Exercise 4 Preparing a Buffer

How many moles of NH4Cl must be added to 2.0 L of 0.10 M NH3 to form a buffer whose pH is 9.00?(Assume that the addition of NH4Cl does not change the volume of the solution.)

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When Strong Acids or Bases Are Added to a Buffer

When strong acids or bases are added to a buffer, it is safe to assume that all of the strong acid or base is consumed in the reaction.

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Addition of Strong Acid or Base to a Buffer

1. Determine how the neutralization reaction affects the amounts of the weak acid and its conjugate base in solution.

2. Use the Henderson–Hasselbalch equation to determine the new pH of the solution.

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Calculating pH Changes in Buffers

Example 5

A buffer is made by adding 0.300 mol HC2H3O2 and 0.300 mol NaC2H3O2 to enough water to make 1.00 L of solution. The pH of the buffer is 4.74. Calculate the pH of this solution after 0.020 mol of NaOH is added.

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Titration

In this technique, a known concentration of base (or acid) is slowly added to a solution of acid (or base).

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Titration

A pH meter or indicators are used to determine when the solution has reachedthe equivalence point, at which the stoichiometric amount of acid equals that of base.

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Titration of a Strong Acid with a Strong Base

From the start of the titration to near the equivalence point, the pH goes up slowly.

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Titration of a Strong Acid with a Strong Base

Just before (and after) the equivalence point, the pH increases rapidly.

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Titration of a Strong Acid with a Strong Base

At the equivalence point, moles acid = moles base, and the solution contains only water and the salt from the cation of the base and the anion of the acid.

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Titration of a Strong Acid with a Strong Base

As more base is added, the increase in pH again levels off.

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Exercise 6 Calculating pH for a Strong Acid–Strong Base Titration

Calculate the pH when (a) 49.0 mL and (b) 51.0 mL of 0.100 M NaOH solution have been added to 50.0 mLof 0.100 M HCl solution.

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Titration of a Weak Acid with a Strong Base

• Unlike in the previous case, the conjugate base of the acid affects the pH when it is formed.

• At the equivalence point the pH is >7.

• Phenolphthalein is commonly used as an indicator in these titrations.

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Titration of a Weak Acid with a Strong Base

At each point below the equivalence point, the pH of the solution during titration is determined from the amounts of the acid and its conjugate base present at that particular time.

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Titration of a Weak Acid with a Strong Base

With weaker acids, the initial pH is higher and pH changes near the equivalence point are more subtle.

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Exercise 7 Calculating pH for a Weak Acid–Strong Base Titration

Calculate the pH of the solution formed when 45.0 mL of 0.100 M NaOH is added to 50.0 mL of 0.100 M CH3COOH (Ka = 1.8 105).

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Exercise 8 Calculating the pH at the Equivalence Point

Calculate the pH at the equivalence point in the titration of 50.0 mL of 0.100 M CH3COOH with 0.100 M NaOH.

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Titration of a Weak Base with a Strong Acid

• The pH at the equivalence point in these titrations is <7, so using phenolphthalein would not be a good idea.

• Methyl red is the indicator of choice.

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Titrations of Polyprotic Acids

When one titrates a polyprotic acid with a base there is an equivalence point for each dissociation.

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Solubility Products

Consider the equilibrium that exists in a saturated solution of BaSO4 in water:

BaSO4(s) Ba2+(aq) + SO42(aq)

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Solubility Products

The equilibrium constant expression for this equilibrium is

Ksp = [Ba2+] [SO42]

where the equilibrium constant, Ksp, is called the solubility product.

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Solubility Products

• Ksp is not the same as solubility.

• Solubility is generally expressed as the mass of solute dissolved in 1 L (g/L) or 100 mL (g/mL) of solution, or in mol/L (M).

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Exercise 9 Calculating Ksp from Solubility

Solid silver chromate is added to pure water at 25 C, and some of the solid remains undissolved. The mixture is stirred for several days to ensure that equilibrium is achieved between the undissolved Ag2CrO4(s) and the solution. Analysis of the equilibrated solution shows that its silver ion concentration is 1.3 104 M. Assuming that Ag2CrO4 dissociates completely in water and that there are no other important equilibria involving Ag+ or CrO4

2– ions in the solution, calculate Ksp for this compound.

AqueousEquilibria

Exercise 10 Calculating Solubility from Ksp

The Ksp for CaF2 is 3.9 1011 at 25 C . Assuming that CaF2 dissociates completely upon dissolving and that there are no other important equilibria affecting its solubility, calculate the solubility of CaF2 in grams per liter.

AqueousEquilibria

Factors Affecting Solubility

• The Common-Ion Effect– If one of the ions in a solution equilibrium

is already dissolved in the solution, the equilibrium will shift to the left and the solubility of the salt will decrease:

BaSO4(s) Ba2+(aq) + SO42(aq)

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Exercise 11 Calculating the Effect of a Common Ion on Solubility

Calculate the molar solubility of CaF2 at 25 C in a solution that is (a) 0.010 M in Ca(NO3)2, (b) 0.010 M in NaF.

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Factors Affecting Solubility• pH

– If a substance has a basic anion, it will be more soluble in an acidic solution.

– Substances with acidic cations are more soluble in basic solutions.

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Exercise 12 Predicting the Effect of Acid on Solubility

Which of these substances are more soluble in acidic solution than in basic solution: (a) Ni(OH)2(s), (b) CaCO3(s), (c) BaF2(s), (d) AgCl(s)?

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Factors Affecting Solubility• Complex Ions

– Metal ions can act as Lewis acids and form complex ions with Lewis bases in the solvent.

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Factors Affecting Solubility

• Complex Ions– The formation

of these complex ions increases the solubility of these salts.

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Factors Affecting Solubility• Amphoterism

– Amphoteric metal oxides and hydroxides are soluble in strong acid or base, because they can act either as acids or bases.

– Examples of such cations are Al3+, Zn2+, and Sn2+.

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Will a Precipitate Form?

• In a solution,– If Q = Ksp, the system is at equilibrium

and the solution is saturated.

– If Q < Ksp, more solid can dissolve until Q = Ksp.

– If Q > Ksp, the salt will precipitate until Q = Ksp.

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Exercise 13 Predicting Whether a Precipitate Forms

Does a precipitate form when 0.10 L of 8.0 103 M Pb(NO3)2 is added to 0.40 L of 5.0 103 M Na2SO4?

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Selective Precipitation of Ions

One can use differences in solubilities of salts to separate ions in a mixture.

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